Driven5 wrote:
At least as far as mufflers go, bigger is better is generally a good philosophy.
Personally, I'm a big fan of having a cat in the exhaust too. It's pretty much all benefits, including quieting things a bit. There's really no reason not to on any modern EFI engine. They make some pretty small ones that might still be able to fit in the remaining space as well.
Any engine feature that potentially dumps unburnt fuel into the exhaust will ruin cats. Launch control, traction control, flat shift, antilag, and more can utilize a spark cut that spits fuel into the cats.